Brain size can vary tremendously between species. For instance, the average human head holds around three pounds of cerebral matter; your typical chimp, on the other hand, packs about a third of that.

Differences in brain size do exist within species (women, for example, have relatively larger brains than men), however such variations are usually small. Not so for the three-spined stickleback. Cloistered away in Iceland's Lake Mýatn, a tiny population of these torpedo-shaped fishes is one of the only known examples on Earth where the males of the species possess dramatically larger brains than females.

In a study published in the latest issue of PLoS ONE, a team of researchers led by Alexander Kotrschal of Sweden's Uppsala University weighed the brains of 58 male sticklebacks and 61 females. While all of the specimens were around 4.5 centimeters in length, the typical male brain weighed in at 24.2 milligrams, while the females' weighed an average of just 19.7 mg.

The underlying reason for this disparity — clearly noticeable in the male and female stickleback brains seen here — is unknown, but the researchers hypothesize that it has to do with two of most important activities in nature: sex and parenting. Male sticklebacks from the Mývatn population have been the subject of behavioral investigations for decades on account of their complex mating rituals and fastidious child-rearing habits, while females are known to dedicate up to 40 percent of their body mass to their reproductive organs.

Korschal and her colleagues suggest that the dramatic difference in brain size is generated in part by the cognitively demanding challenges faced by males in impressing their mates and raising their young. But the gap could also be widened by the possibility that females, in diverting so much energy toward their baby-making bits, sacrifice investments toward energetically expensive brain tissue.

But these kinds of gender-specific trade-offs abound in the animal kingdom (just look at male bats, who often trade brain mass for bigger balls); so why haven't differences this dramatic been observed in any other organisms? The answer will lie in future analyses on differences in brain size across numerous species, families, and classes of animals. Such studies, say the researchers, could open up brand new fields of research for understanding brain evolution.